One in five Mac computers is likely to carry Windows malware, but only one in 36 is likely to be infected with malware specifically designed for the Mac OS X, according to study performed by antivirus firm Sophos.

Sophos collected malware detection statistics from 100,000 Mac computers that run its free antivirus product and found that 20 percent of them contained one or more types of Windows malware.

When stored on a Mac, Windows malware is inactive and can't do any harm, unless that computer has Windows installed as a secondary OS.

However, such malicious files can still be transferred unknowingly by Mac users to Windows machines via file sharing, USB memory sticks, external hard disk drives and other removable media devices.

Sophos' analysis also revealed that 2.7 percent of the 100,000 scanned Macs were actually infected with Mac OS X malware and a large part of those infections, 75 percent, were with the Flashback Trojan.

Flashback is a family of Mac OS X malware distributed through social engineering and automated Web exploits. Sophos' products detect applications from this malware family as OSX/Flshplyr.

A recent Flashback variant that appeared at the end of March and spread by exploiting a vulnerability in the Java browser plug-in, managed to infect almost 700,000 Mac computers.

Around 650,000 Macs are still infected with it, despite Apple releasing a patch for the Java vulnerability and a Flashback removal tool, according to a report released on Friday by antivirus firm Doctor Web.

The second most common type of malware detected by Sophos' Mac antivirus product was OSX/FakeAV, with 18 percent of the total. OSX/FakeAV is a family of Mac OS X scareware applications that includes fake antivirus programs like Mac Defender, which first appeared in May 2011.

OSX/RSPlug, a Mac OS X version of the DNSChanger computer Trojan, was the third most common detection and accounted for 5.5 percent of the total. This malware forces infected computers to use rogue DNS (Domain Name System) servers controlled by attackers.

The rogue DNS servers used by the DNSChanger botnet were seized by the FBI last year and were temporarily replaced with good ones, to allow the malware's victims to clean their computers.

The replacement servers are scheduled to be shut down on July 9, but according to the FBI, there are still 350,000 computers infected with the malware. If the servers are shut down, those computers will no longer be able to access the Internet.

"Some Apple fans might feel relieved that they are seven times more likely to have Windows malware on their Macs than Mac OS X-specific threats, but they shouldn't be," said Graham Cluley, a senior technology consultant at Sophos, in a blog post on Tuesday. "What Mac users really need to do is protect their computers now (there really is no excuse, free anti-virus software is available for Mac home users), or risk allowing the malware problem on Macs to become as big as the problem on PCs in the future."

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